With the quality of coatings on new lenses I suspect lighting conditions need to be pretty crazy for the hood length to be very significant from an IQ standpoint. Weather conditions may be a bigger factor. If you're a fair-weather photographer, the a shorter hood is likely fine. If you go out in rain, mist, and snow, a longer hood will help keep the elements off your front element.
An early customer shoots speed runs on the Bonneville Salt Flats, I designed an as-long-as-possible hood for his 400 f/2.8. The white salt in full sun is pretty wicked lighting. The customer reported improved image contrast and improved AF with the XL hood. Full sun in snow could be similar.